Keep track of last matrix activity

[tzarc]

Description

Re-submission of #10730 -- there was a logic issue that only seemed to manifest on some AVR builds.
@daskygit has graciously performed the investigation on their end, as I was unable to reproduce.

This PR adds support for recording the last time matrix activity was detected.
Two new APIs have been added:

uint32_t last_matrix_activity_time(void);     // Timestamp of the last matrix activity
uint32_t last_matrix_activity_elapsed(void);  // Number of milliseconds since the last matrix activity

These values are compatible with normal matrix scanning, as well as split_common.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[tzarc]

Ignore the draft toggle -- mistakenly thought I'd lost the changes for non-split code... they were there already.